Abstract OBJECTIVE: To determine the prevalence of upper respiratory tract colonization by Moraxella catarrhalis in children under six years of age. MATERIAL AND METHODS: A survey was conducted between January and December 1998 in Mexico City, amongchildren aged 2 months to 5 years, selected through cluster sampling. Pharyngeal samples were taken for M. catarrhalis identification.Theminimal inhibitory concentration to different antibiotics was obtained and beta-lactamases were determined by the iodometric test. Statistical analysis consisted of frequency distributions, odds ratios, 95% confidence intervals, and Mantel-Haenszel c2 . Statistical significance was set at p<0.05. RESULTS: After excluding 37 children, the study population was 604 children from Mexico City; M. catarrhalis was present in 130 pharyngeal specimens (22.9%). Most of the strains were positive for beta-lactamase production (75.4%). Eighty percent of the strains was resistant to penicillin and 70% to ampicillin and amoxicillin. None were resistant to cefotaxime, imipenem, meropenem and erythromycin. CONCLUSIONS: Prevalence of M. catarrhalis upper respiratory tract colonization is similar to that of other respiratory pathogens. These findings warrant future research on the role of M. catarrhalis as an etiologic agent in acute and chronic respiratory infections in Mexico. The English version of this paper is available at: http://www.insp.mx/salud/index.html Key words: Moraxella (Branhamella) catarrhalis/prevalence; respiratory tract infections/carrier stak; beta-lactam resistance; Mexico